Speaker 1: Welcome to brain Stuff, a production of I Heart Radio,
Speaker 1: Hey brain Stuff Lauren Vogel Bam. Here, sharks take doing
Speaker 1: the wave to a whole new level. They swim by
Speaker 1: waving their body inside to side curves. It starts with
Speaker 1: the head turning first one way, then another. The movement
Speaker 1: ripples down their torpedo shaped body, pushing the water away
Speaker 1: and providing forward propulsion. Last comes the tail, with the
Speaker 1: fins shaped to allow them to move quickly and achieve
Speaker 1: immense liftoff or propulsive downward thrust. Obviously, a shark has
Speaker 1: to swim to catch its prey, meet its mates, and
Speaker 1: avoid its predators. But does a shark have to swim
Speaker 1: just to stay alive. You may have heard that a
Speaker 1: shark will drown if it stops moving, an idea that's
Speaker 1: been sited everywhere from biology textbooks to Ripley's Believe it
Speaker 1: or not. This theory came about by comparing sharks, which
Speaker 1: are classified as cartilaginous fish, to bony fish, which have
Speaker 1: many more muscles around their breathing apparatus is the gills.
Speaker 1: To understand why this distinction is important, let's take a
Speaker 1: look at how sharks and other fish breathe. To breathe,
Speaker 1: sharks must remove oxygen from the water around them. The
Speaker 1: water enters the shark's mouth, its nose being used exclusively
Speaker 1: for smell, and then the water flows over the gills.
Speaker 1: Inside those gills, there are hundreds of feathery gill filaments.
Speaker 1: Each filament, in turn has thousands of leaf like lamellie
Speaker 1: or flaps, which contain blood vessels. The blood absorbs the
Speaker 1: oxygen from the incoming water, and the excess water flows
Speaker 1: back out of the shark's body through gill slits. Sharks
Speaker 1: have five to seven pairs of gill slits, depending on
Speaker 1: the species. Using this method, sharks can extract about eighty
Speaker 1: per cent of oxygen out of the meager one percent
Speaker 1: of free oxygen that's present in the water. To compare,
Speaker 1: the air that we breathe is made up of about
Speaker 1: available oxygen, but human lungs only extract about of that anyway,
Speaker 1: to maintain a steady flow, the shark constantly needs to
Speaker 1: be taking in water. But does it have to constantly
Speaker 1: swim to take in this water? A scientist's thought so,
Speaker 1: because other fish seemed to have the equipment to actively
Speaker 1: pump the water through their mouth and over their gill slits,
Speaker 1: whereas sharks looked less developed. But that doesn't account for
Speaker 1: sharks that just kind of chill, like angel sharks and
Speaker 1: nurse sharks. It turns out that not all sharks have
Speaker 1: to stay moving to breathe. The oldest sharks, the ancestors
Speaker 1: of more modern sharks, didn't have to constantly swim in
Speaker 1: order to breathe. Rather, they pumped water through their mouth
Speaker 1: and over their gills. This method is known as buckle pumping,
Speaker 1: named for the buckle or cheek muscles that pull the
Speaker 1: water into the mouth and over the gills. Many sharks
Speaker 1: retain this method today, such as the aforementioned nurse and
Speaker 1: angel sharks, as well as carpet sharks, eskates, and rays.
Speaker 1: The sharks cousins also breathe this way. These species tend
Speaker 1: to spend most of their time laying on the bottom
Speaker 1: of the ocean floor. Many of these sharks are dorso
Speaker 1: ventrally flattened, that is, sort of squashed looking along the
Speaker 1: length of their Backlike the angel shark, they have stronger
Speaker 1: muscles in the face. These sharks might also have a
Speaker 1: more prominent spircle, which is a tube behind the eyes.
Speaker 1: When a shark is buried at the bottom of the
Speaker 1: ocean floor and can't breathe through its mouth, the spircle
Speaker 1: can pull in water instead. As sharks evolved and became
Speaker 1: more active, however, buckle pumping became secondary. It was simply
Speaker 1: more energy efficient to take in water while swimming, in
Speaker 1: effect ramming the water into the mouth and letting it
Speaker 1: flow through the gill slits. This method of breathing is
Speaker 1: known as a ram ventilation. Most sharks can alternate between
Speaker 1: buckle pumping and ram ventilation depending on what they're doing.
Speaker 1: When they start swimming fast enough to force the water
Speaker 1: in more quickly than they could pump it, then they
Speaker 1: stop pumping. The sand tiger shark is an example of
Speaker 1: a shark that switches back and forth. Some sharks, however,
Speaker 1: have completely lost the ability to breathe by buckle pumping,
Speaker 1: and these are the sharks that will indeed drown if
Speaker 1: they stop swimming and thus ramming water. These sharks are
Speaker 1: known as obligate ram breathers or obligate ram ventilators, and
Speaker 1: only about two dozen of the four hundred identified shark
Speaker 1: species are required to maintain this forward swimming motion. These
Speaker 1: include the great white shark, the mako shark, the salmon shark,
Speaker 1: and the whale shark. So do these obligate ram breathers
Speaker 1: ever get a break? Aren't they tired? As it turns out,
Speaker 1: it's more work for these sharks to remain still than
Speaker 1: it is to swim. In a study of lemon shark
Speaker 1: switch switch between breathing methods, juveniles breathed six percent more
Speaker 1: efficiently when moving than when resting, even when resting so
Speaker 1: that the current allowed the water to flow directly into
Speaker 1: their mouths. That's not to say that these sharks don't
Speaker 1: catch a break every now and then. For obvious reasons,
Speaker 1: it can be hard to keep track of a shark
Speaker 1: that's constantly swimming, so it's difficult for scientists to know
Speaker 1: how or when they rest. An experiment with a small shark,
Speaker 1: the spiny dogfish, indicated that swimming is coordinated by the
Speaker 1: spinal cord, not the brain, so sharks may be able
Speaker 1: to shut down their brain and rest while still swimming.
Speaker 1: Sharks in needisome rest may also take advantage of the
Speaker 1: factors that affect the amount of oxygen in the water,
Speaker 1: such as salinity, temperature, and even the time of day.
Speaker 1: In the nineteen seventies, scientists investigated what came to be
Speaker 1: known as the Caves of the Sleeping Sharks in Mexico.
Speaker 1: Inside the caves were motionless reef sharks, which are normally
Speaker 1: obligate ventilators. The scientists determined that the water in the
Speaker 1: caves had an extremely high amount of oxygen and reduced salinity.
Speaker 1: These conditions likely made it easier for even these sharks
Speaker 1: to breathe without moving. They may not have been asleep
Speaker 1: like humans, their eyes were open, for one thing, but
Speaker 1: it does appear that sharks can get some rest, and
Speaker 1: many other reef sharks have also been observed motionless at
Speaker 1: the bottom even outside of caves. Scientists still aren't exactly
Speaker 1: sure how they can do this. Today's episode is based
Speaker 1: on the article will a shark drown if it stops moving?
Speaker 1: On how stuffworks dot Com, written by Molly Edmonds. Brain
Speaker 1: Stuff is production of I Heart Radio in partnership with
Speaker 1: how stuff works dot Com and is produced by Tyler Klang.
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